The chemical literature discloses numerous processes for coupling nucleosides through phosphorous-containing covalent linkages to produce oligonucleotides of defined sequence. One of the most popular processes is the phosphoramidite technique (see, e.g., Beaucage, et al., Tetrahedron 1992, 48, 2223 and references cited therein), wherein a nucleoside or oligonucleotide having a free hydroxyl group is reacted with a protected cyanoethyl phosphoramidite monomer in the presence of a weak acid to form a phosphite-linked structure. Oxidation of the phosphite linkage followed by hydrolysis of the cyanoethyl group yields the desired phosphate or phosphorothioate linkage.
The phosphoramidite technique, however, is not without its disadvantages. For example, cyanoethyl phosphoramidite monomer is quite expensive. Although considerable quantities of monomer go unreacted in a typical phosphoramidite coupling, unreacted monomer can be recovered, if at all, only with great difficulty. Also, acrylonitrile, the by-product of deprotection of the cyanoethoxy group on the phosphate group is carcinogenic and in some cases acts as a Michael acceptor to form undesired side-products.
Consequently, there remains a need in the art for synthetic methods that will overcome these problems.